1. Field of the Invention
The present invention relates to a ladder type band-pass filter, more particularly, to a ladder type band-pass filter comprising plural vibrator blocks, each vibrator block being produced as a chip.
2. Description of Related Art
Conventionally, there has been put into practical use, a ladder type band-pass filter comprising ceramic piezoelectric vibrators 15 shown in the equivalent circuit diagram of FIG. 1, which has a steep cut-off frequency characteristic.
FIG. 2 shows the appearance of a ladder type band-pass filter comprising piezoelectric vibrators, and FIG. 3 shows a cross section thereof.
Referring to FIG. 3, the piezoelectric vibrators 15a, 15b, 15c, 15d, 15e, 15f and 15g are supported for vibration by spring terminal plates 14 and insulating plates 13 between opposing side plates 11a and 11b which are formed on a base 11 of an electrically insulating resin, and the piezoelectric vibrators 15a, 15b, 15c, 15d, 15e, 15f and 15g are electrically connected through lead wires so as to form a ladder type band-pass filter between an input terminal 16 and an output terminal 17 which are formed integrally in the base 11. Thereafter, the base 11 comprising the piezoelectric vibrators 15a, 15b, 15c, 15d, 15e, 15f and 15g is inserted and fixed in a metallic case 18, and then, a bottom portion of the metallic case 18 is sealed by a sealing member 19. In FIGS. 2 and 3, 20a and 20b denote ground terminals. It is to be noted that a resin case may be used instead of the metallic case 18.
In the conventional ladder type band-pass filter, the dimensions of the base 11 and the metallic case 18 are changed depending on the frequency of the pass band thereof and the number of the piezoelectric vibrators 15a, 15b, 15c, 15d, 15e, 15f and 15g. Therefore, it is necessary to prepare the other metal molds for forming the base 11 and the metallic case 18, and further, it becomes difficult to assemble the ladder type band-pass filter, automatically, resulting in a higher production cost.
Today, almost all the electric devices and components such as capacitors and resistor are produced as chips in order to improve the productivity and to decrease the production cost thereof. However, vibrators of conventional ladder type band-pass filters have not yet been produced as a chip.